1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) filter, and in particular, to an SAW filter for high frequency that realized an acute skirt characteristic and resistance to input wave of high power by employing a carbon nanotube (CNT) as an acoustic wave transmitting media of the SAW filter as well as by enlarging the space between electrodes of inter-digital transducers.
The present invention also relates to a method for manufacturing an SAW filter that simplifies a manufacturing process by omitting a surface abrasive process, shortens a manufacturing process, and lowers a manufacturing cost due to the short manufacturing process by using a tetrahedral amorphous carbon (ta-C) as a transmitting medium of the SAW, while facilitating formation of a wide area and reducing transmission loss and noise.
2. Description of the Prior Art
An SAW filter is an element used for removing noise and interference through selective pass of the bandwidth of a particular frequency (center frequency) by using piezoelectric characteristics that are inter-transducible between electricity and machinery. SAW filters are used for core parts of TV, VCR, mobile telecommunications, etc. as bandpass filters.
FIG. 1 is a schematic diagram of an SAW filter in general.
As shown in FIG. 1, the SAW is an element for transducing electric signals to acoustic wave along the surface of a piezoelectric substrate 103 by taking an advantage of a total distortion effect (an inverse piezoelectric effect) of the substrate through installation of two inter-digital transducers IDT 101, 102 on the piezoelectric substrate, and detecting the generated SAW as an electric signal by means of an output transducer.
Here, the IDT comprises a transmitting IDT 101 and a receiving IDT 102. The transmitting IDT 101 transduces the electric signals inputted to a signal generator to an SAW. The transduced SAW progresses along the surface of the piezoelectric substrate 103 so as to be transmitted to the receiving IDT 102.
As a consequence, the receiving IDT 102 transduces the SAW signals to electric signals again by using the piezoelectric effect. Here, the receiving IDT 102 performs a wave filtering so that frequency characteristics can be determined by a geometrical structure of the IDT electrode.
FIG. 2 is a graph illustrating frequency pass characteristics of the SAW filter in general.
As shown in FIG. 2, the SAW filter is set to pass transmitted signals only of a particular frequency, and used as a bandpass filter with set amplitude and phase.
Also, the materials most widely used for a substrate of the SAW filter are LiTaO3 (LTO) and LiNbO3 (LNO), which are mono-crystal materials. However, the SAW filter using such mono-crystal materials cannot be used for high frequency greater than GHz. Therefore, new materials are being developed in a large scale.
Meanwhile, the center frequency of the SAW is proportional to a velocity of the SAW in a transmission medium but inverse proportional to the spacing between the IDT electrodes. Therefore, it is necessary to either use a material of high elasticity or narrow the spacing between the IDT electrodes to heighten the frequency band of the center frequency.
However, there exists a limitation to reduce the spacing between IDTs due to the limitation of patterning technology of IDT electrodes deriving from the limitation of lithography technique as well as to the problems in securing stability and durability against high pressure power. For these reasons, diverse researches are being conducted in a manner of introducing a medium of high elasticity.
Also, in the basic SAW filter, piezoelectric material was a transmitting medium that takes charge of piezoelectric effect and total distortion effect (inverse piezoelectric effect). However, no piezoelectric material of high elasticity has been developed to date in light of the mechanical elastic characteristics of the piezoelectric material. Therefore, new media are being introduced to transmit the SAW.
Outstanding examples of that case are a xe2x80x9cpiezoelectric material (mainly ZnO)/diamond filmxe2x80x9d structure, a xe2x80x9cpiezoelectric material (mainly ZnO)/sapphirexe2x80x9d structure, and an xe2x80x9cLiNbO3/diamondxe2x80x9d structure that have introduced either diamond or sapphire as an SAW medium.
With this introduction, a notably higher transmission velocity (5,200-5,700 m/sec in case of sapphire, and 9,000-11,900 m/sec in case of diamond) than the existing Quartz (3,158 m/sec), LNO (3,488 m/sec) etc. could be achieved. Thus, filters became available in higher frequency band.
While employing diamond for a medium of the SAW has an advantage of availing the SAW filter in high frequency band, however, it also poses the following problems.
First, the process of synthesizing diamond requires high temperature. Also, because of a deflection of the substrate caused by stress of the diamond, it is difficult to realize an area to be greater than 4 inches, for instance.
Further, since the diamond is poly-crystal, a grain boundary exists, thereby increasing the electric wave loss of the signal. Also, despite the need to undergo abrasion of the rough surface of the diamond for use as a medium of the SAW, it is difficult to perform the abrasion process due to hardness of the diamond as well as to consumption of time and expense.
It is, therefore, an object of the present invention to provide an SAW filter for high frequency that realized an acute skirt characteristic and resistance to input wave of high power as well as to high frequency higher than 2 GHz by employing a carbon nanotube as an acoustic wave transmitting media of the SAW filter by enlarging the space between electrodes of inter-digital transducers.
To achieve the above object, there is provided an SAW filter according to one aspect of the present invention, comprising: a substrate; a complex film composed of CNT and a piezoelectric material, and having a piezoelectric characteristic so as to transmit SAW; and IDTs for outputting electric signals by receiving the SAW from the complex film.
There is also provided a method for manufacturing an SAW filter, comprising the steps of: forming a complex film on a substrate; forming a conductive material on the complex film; and patterning the conductive material.
Here, the complex film is formed either of CNT and a piezoelectric material or of a complex comprised of CNT and an insulator and a piezoelectric material formed on the complex.
The method for forming the complex film according to one aspect of the present invention comprises the steps of: forming catalyst patterns by means of a lithography technique used in an ordinary semiconductor process; growing a CNT bridge in horizontal direction between the catalyst patterns; and forming a piezoelectric material on the grown CNT bridge by sputtering.
The method for forming the complex film according to another aspect of the present invention comprises the steps of: forming suspension by dispersing the CNT into a predetermined solution; attaching the CNT suspension to the substrate by means of electromagnetic field or electrostatic force; and forming a piezoelectric material on the substrate, to which the CNT suspension has been attached, by sputtering.
The method for forming the complex film according to another aspect of the present invention comprises the steps of: forming suspension by dispersing the CNT into a predetermined solution; precipitating the CNT suspension by filtering through a filter membrane; and forming a piezoelectric material on the substrate, in which the CNT has been precipitated, by sputtering.
In the process of filtering the CNT suspension through the filter membrane, the CNT suspension is aligned by means of magnetic field.
There is also provided a SAW filter according to another aspect of the present invention, comprising: a substrate; an acoustic wave transmission medium of SAW composed of tetrahedral amorphous carbon and formed on the substrate so as to transmit SAW; a piezoelectric material having piezoelectric characteristics and formed tightly onto the SAW; and IDTs for generating SAW by transmitting inputted electric signals to the piezoelectric material, and outputting the electric signals by receiving the SAW from the piezoelectric material.
Here, the acoustic wave transmission medium of the tetrahedral amorphous carbon formed on the substrate is deposited by arc discharging with respect to black lead. The acoustic wave transmission medium of the tetrahedral amorphous carbon formed on the substrate has a thickness less than 1 xcexcm.
The acoustic wave transmission medium of the tetrahedral amorphous carbon formed on the substrate is deposited by laser ablation using black lead.
Thus, the present invention described as above can provide an SAW filter for high frequency that has an acute skirt characteristic and is resistant to an input wave of high power by employing a CNT as an acoustic wave transmission medium of the SAW filter and by enlarging the space between electrodes of inter-digital transducers.
The present invention relates to an SAW filter employing a CNT (having an elasticity of 1.8 Tpa), which is known to be a material of highest elasticity as an acoustic material of the SAW filter. The CNT can be produced to have a variety of elasticity depending on the structure and diameter thereof.